Enamel is the hardest material in vertebrates and is the most highly mineralized skeletal tissue present in the body. Mature enamel, considered the most resistant and tough material in the biological world, is composed of carbonate hydroxyapatite (CHA) (95-97% wt) and less than 1% wt of organic material. Unlike other biomineralized tissues, such as bone and dentin, mature enamel does not contain cells and therefore cannot be regenerate itself and therefore cannot be biologically remodeled. Consequently, enamel regeneration cannot take place in vivo. There is no biological process that can repair degraded or damaged enamel, evidencing the need for synthetic enamel biocompatible materials able to repair teeth decay.
Enamel makes up the outermost layer of the tooth crown having a thickness of about 1-2 mm and containing a high mineral content which imparts to the enamel a high modulus, but also making it susceptible to cracking. Dentine lies below the enamel and is tougher, forming the bulk of the tooth and absorbing stresses from enamel, preventing its fracture.
The mechanisms involved in the damage of dental hard tissue are related to the acid attacks on the outer few micrometers of the enamel, with the consequent demineralization and dissolution of the minerals.
Frequent application of a high concentration of topical fluoride may be of some benefit in preventing further demineralization and increasing the abrasion resistance of erosion lesions. In-vitro studies have shown that inhibition of dissolution of synthetic carbonated hydroxyapatite is a logarithmic function of the fluoride concentration in solution.
Most of the products and devices commonly used to counter enamel and dentine erosion such as fluoride, work by reducing apatite dissolution and increasing surface micro hardness, but are unable to reconstruct the lost mineral.
In vitro, fluoride (0.02-0.10 mg/L) addition to a supersaturated solution of calcium phosphate induces the crystallization of hydroxyapatite, Ca10(PO4)6(OH)2 which is the mineral phase of bone and teeth. Increasing fluoride concentration fluoroapatite Ca10(PO4)6F2 is formed and appears in more ordered and bigger apatite crystals which are less soluble in an acidic environment.
In the body, in vivo fluoride is mainly associated with calcified tissue, bone and teeth due to its high affinity to calcium. Fluoride modifies the bone mineral phase by replacing the hydroxyl groups in the hydroxyapatite phase producing its partial conversion into fluoroapatite. The increased electrostatic stability and crystallinity of flouride substituted hydroxyapatite increases the bone density and hardness reducing the mechanical strength.
The use of fluoride-containing oral care products are recommended especially in countries where the fluoride concentration in drinking water is low even if there are considerable differences regarding the starting time (birth-6 months of age) and amounts related to age. Fluoride is not essential for human growth and development and its content in the body is not under physiological control. Adsorbed fluoride is rapidly distributed by circulation to the intracellular and extra cellular fluid, but is retained only in calcified tissues.
Fluoride accumulation in skeleton changes bone mechanical behaviour reducing bone strength and increasing its density and stiffness, causing skeletal deformities and risk of fractures. Fluoride is not irreversibly bound to bone and can be released during remodelling of bone.
In adults, adsorbed fluoride is only partially less than 50% retained in skeleton and the remainder excreted prevalently via the kidney. On the contrary, in infants fluoride retention in bone can be as high as 90% and appears also incorporated into dental enamel during teeth formation. Excessive intake of fluoride during enamel maturation from birth to eight years of age, when enamel formation is complete, can lead to reduced mineral phase content of enamel and to dental fluorosis of deciduous, but prevalently of permanent teeth.
For these reasons, toothpastes containing a very high fluoride concentration exceeding the International Standards Organization limits are exclusively prescribed by professionals and are not recommended for children. The European Food Safety Authority (EFSA) Scientific Panel considers that the maximum fluoride intake should be of 0.1 mg fluoride/kg/day in children aged 1-8 years which is equivalent to 1.5 and 2.5 mg fluoride per day in children aged 1-3 years and 4-8 years respectively.